The present invention relates to a safety belt apparatus for vehicles. In particular, the invention relates to a safety belt tensioner.
A typical safety belt apparatus may include a safety belt and a belt reel. The belt reel, which may take up a greater or lesser proportion of the belt, is fixed rotatably on the vehicle chassis and is preloaded in the direction of belt winding by a torque-producing mechanism, typically a spiral spring. The reel may include a belt unwinding blocking arrangement, that blocks the unwinding of the belt against the force of the torque-producing mechanism if there is an attempt to pull the belt out quickly. The blocking arrangement also preferably acts to block the unwinding of the belt when an acceleration relating to an accident is sensed. Furthermore, the apparatus may include a belt redirection device, which is typically arranged above the shoulder of occupants held by the safety belt. This is fed by the belt reel to the redirection device and may be redirected toward the occupant. The apparatus may also include a belt buckle, to which the belt extends from the belt apparatus and which is fixed on a tension member mounted on a vehicle chassis. A typical example of the structure described above is disclosed in DE 199 15 024 (incorporated by reference herein in its entirety).
Since the safety belt rests only relatively loosely on the occupant owing to the action of the torque-producing mechanism on the belt reel, belt tensioners (i.e. pretensioners) are often used. A belt tensioner acts to tension the safety belt abruptly in the event of an accident so that it comes to rest firmly against the occupant. Belt tensioners of this kind normally act on the belt reel. The belt reel is typically rapidly rotated in the belt winding direction by a pyrotechnic charge, for example, if when acceleration due to an accident is sensed to be occurring. An example of such a structure is disclosed in EP 581 288 B1 (incorporated by reference herein in its entirety.
DE 199 57 794 A1 (incorporated by reference herein) discloses a safety belt arrangement in which the belt redirection apparatus is also used for belt tensioning by appropriately displaceable arrangement on the vehicle chassis.
Safety belt arrangements with belt-force limiters are also known. In principle, limitation of the belt force is achieved through the twisting of a torsion bar, typically located within the reel structure. Such belt-force limiters generally have a flange, a spindle for winding and unwinding the belt, and a torsion bar. In the event of an accident, the flange is connected to the body of the motor vehicle with the aid of a locking apparatus in a manner that prevents twisting. In certain circumstances, it is possible for the belt wound onto the spindle to be unwound with a limited force resulting from the torsional moment of the torsion bar.
One disadvantage of the above-mentioned design is that the torsion bar requires a certain angle of twist to reach its maximum level of counter force, at which point limitation of the belt tension in a manner optimum for occupant protection is achieved. During the twisting of the torsion bar through this certain angle of twist, the occupant is not restrained with the maximum possible shoulder force. As a result, increased forward displacement of the vehicle occupant may occur.